The significant representative of the endogenic prostaglandins discovered in 1976 i.e. prostacycline (PGI.sub.2) shows a considerable biological activity (Prostacyclin, ed.: J. R. Vane and S. Bergstrom Raven Press, N.Y. 1979).
From a therapeutical point of view the blood aggregation inihibiting, the peripheric vasodilative and cytoprotective activity of the prostacycline are of particular importance. The extraordinary instability of prostacycline creates a limit to the therapeutic applicability of these useful biological properties. Its half life in aqueous solutions at neutral pH is only 3.5 minutes. More useful than prostacycline are those therapeutically active substances which are much more stable than prostacycline and preserve biological activity.